The traditional mechanism to achieve autofocus in miniature cameras, and all cameras in general, is based on translating the lens barrel along the optical axis (i.e. z axis) to continuously keep the projected image in focus at the image sensor plane. In such autofocus systems, an actuator is required to displace the lens barrel of typically more than 45 mg in mass a stroke of up to 100 microns or more depending on the optical design of the lenses. Different inventions present autofocus cameras using this type of lens barrel actuation mechanism. These include US patents No. U.S. Pat. Nos. 7,285,879 B2/ 7,880,982 B2/ 7,663,818 B2/ 7,586,702 B1/ 7,271,511 B2/ and a number of others. The lens barrel actuation-based autofocus mechanism has a number of drawbacks such as slow speed, large size modules, and optical lens tilt. The speed of the AF mechanism is usually compromised due to the large load of the whole barrel to be displaced by the actuator, i.e. >45 mg. Thus, the actuators used to displace lens barrels are often large in size in order to be able to handle the large weight of the barrel and also to accommodate the size of the lens barrel itself (i.e. lens barrels have a cylindrical shape with a diameter of about 5 mm and a height of almost 3.5 mm or more depending on the specific optical design of the lenses). Furthermore, mounting a lens barrel within the moving stage of the actuator typically introduces an undesired lens tilt of about 0.2° which results in a deterioration of the image quality.
To avoid such drawbacks, a novel mechanism to achieve autofocus in miniature cameras is disclosed in this invention. The new method suggests that the autofocus is achieved by translating the image sensor a stoke of up to 100 microns to keep the projected image on the image sensor plane. The required stroke here is still the same as the required stroke needed to achieve AF when the lens barrel actuation mechanism is used. In both cases, the relative distance between the lenses and the image sensor is to be varied. This method, based on moving the image sensor to achieve autofocus, was not pursued previously possibly due to the fact that there was no actuation technology that could handle the motion of the mass of an image sensor die, which is of the order of tens of milligrams and, at the same time, was compatible with image sensor electrical wiring and packaging. The invention described herein provides for a highly compact autofocus mechanism and the ability to translate the image sensor along the optical axis as well as rotate it along the two axis forming the plane perpendicular to the optical axis so as to correct for any lens tilt that occurs during assembly and/or during use of the camera such as any tilt due to shocks the camera module is subject to.
MEMS piston-tube electrostatic actuators that are developed by the applicants and described in patent application U.S. application Ser. No. 14/449,544 and PCT/IB2014/001498 are able to meet such requirements for autofocus using image sensor actuation.